By default a .NET dll does not need to be registered with the OS like previous DLL's and regsvr...You can however install a .NET dll into the Global Assembly Cache so that it is accesible system wide:
If you want to register a .NET dll so that you can use the .NET classes from a COM environment (like VB6), then you should use the COM Class item template in your VB.NET class library, and use the regasm command line to register the DLL. See http://msdn.microsoft.com/vbrun/vbfusion for details.